The present invention relates to pyrolysis heaters and specifically to an improved burner arrangement to control the heat flux to various sections of the process coils.
A typical pyrolysis heater consists of one or more fireboxes comprising radiant heating sections together with one or more upper convection sections containing feed preheaters. The radiant heating section contains a plurality of radiant process coils suspended in the center plane of the firebox between two radiating walls. The passes of each coil are most often swaged up to gradually larger diameter tubes toward the outlet end. Usually the coils have a number of parallel smaller tubes at the inlet end and fewer larger tubes at the outlet end.
Vertically firing burners located on the hearth or floor of the firebox are used as a heat source inside of many types of pyrolysis heaters. Inside of an ethylene cracking heater, identical hearth burners are spaced on the hearth along both of the long walls of each firebox to provide the high intensity heat release necessary for pyrolysis of the feedstock inside of the process coils. A specific burner design for a particular situation must provide a heat release rate as a function of elevation which is within an acceptable performance envelope. This assures that the process coils receive sufficient heat flux from top to bottom without developing hot spots which promote the formation of deposits inside of the process tubes and reduce the heater availability for production. In a typical pyrolysis heater in an ethylene plant, on the order of eight to ten hearth burners for light feedstocks and perhaps eighteen to twenty for heavy feedstocks are located along each of the refractory walls on the sides of the firebox with the process coil being suspended in the center between the walls. The burners are all of a similar design and they fire upward along the walls at more or less the same rate. This results in the inlet passes and the outlet passes of the process coils being heated at the same flux or heat release rate. Since the gases being treated in the process coils are hotter toward the outlet ends of the coils, these outlet ends are more susceptible to the formation of internal coke deposits. With the inlet and outlet ends of the coils being heated at the same rate, coking is more likely. Further, with hotter process temperatures on the outlet passes and equivalent fluxes, the tube metal temperatures of the outlet passes are normally the highest. In typical radiant coils, the operation is limited by the maximum metal temperature since these expensive alloy tubes operate near their plastic flow limits.
An object of the invention is to heat the process coils of a pyrolysis heater more efficiently and in a manner which will increase the heat flux to the cooler inlet sections and decrease the heat flux to the hotter outlet sections. The object is to reduce the heat flux at the hotter outlet sections to reduce the tendency for coking while still maintaining the required total heat input for cracking. More specifically, the invention involves grouping the inlet sections of the coils together and grouping the outlet sections together and providing high output and low output burners. The burners are arranged and paired to generate a temperature field that is segregated into hotter and cooler zones properly aligned with the specific sections of the process coils. Even more specifically, the invention involves directing the flames from the burners to achieve the desired temperature zones.